An insulated power supply is provided with a transformer to electrically insulate input and output sides. However, if too much magnetic bias occurs in a magnetic core of the transformer, a loss may increase or the magnetic core may be magnetically saturated to generate an excessive excitation current on windings. Necessary solutions to this problem include controlling voltages and detecting a magnetic bias or an excessive DC magnetic flux generated in the magnetic core to eliminate the magnetic bias and the magnetic saturation.
In a magnetic bias evaluation apparatus described in JP-A-H11-40429, the magnetic core of the transformer is provided with a small π-type magnetic core (bypass magnetic path) made of a material whose magnetic permeability is higher than the magnetic core. In addition, the bypass magnetic path is provided with a detection winding. The bypass magnetic path causes magnetic flux saturation more easily than the magnetic core. Even if a magnetic core flux changes, the detection winding induces a voltage only when the magnetic core flux approximates to 0.
If a magnetic bias occurs in the magnetic core whose magnetic flux changes periodically, a time interval of times at which the magnetic core flux becomes zero differs from a time interval in the normal state. Accordingly, a time interval of times at which a voltage is induced to the detection winding also changes. The magnetic bias evaluation apparatus according to JP-A-H11-40429 detects the magnetic bias by monitoring a time interval of times at which a voltage is induced to the detection winding.
However, the magnetic bias evaluation apparatus described in JP-A-H11-40429 requires providing an additional bypass magnetic path for a transformer core, and a cost increases. In recent years, ferrite is generally used as a material of transformer cores. Because such a core is fragile, when the transformer core is provided with the bypass magnetic path, the strength of the transformer core decreases.